In most cases, I try to be a bit careful about it 
and say that I'll contact that person first if I 
should decide to sell, rather than all-out 'dibs' 
as I think that makes it more clear that one 
isn't promising a cel to a specific person, 
rather than just making sure to keep in contact 
with them about a cel.  
In certain cases, I've said flat-out that I'm 
giving somebody preference, and that if I ever 
sell a cel, they're first in line.  
Either way, I take it seriously.  I would be 
disappointed if somebody said the same to me and 
didn't keep their word, and I value integrity.  
One's word may not be a binding contract, but 
when one makes a promise, I feel that they're 
honor bound to take it seriously and keep that 
promise.
It's worked well for me so far; I suppose I just 
feel that if you don't mean it, you shouldn't say 
it.

~Mala